1. Field of the Invention
L-phenylalanine and its methyl ester, L-phenylalanine methyl ester are useful intermediates in peptide synthesis. The demand for these compounds in recent years has increased because they are also starting materials in the synthesis of .alpha.-L-aspartyl-L-phenylalanine methyl ester, a popular sweetener. This invention relates to a novel process for recovering L-phenylalanine from syntheses by formation of an optically active L-phenylalanine monomethylsulfate crystal.
2. Discussion of Background
For convenience, the abbreviations used hereinafter are summarized in the following Table:
______________________________________ Compound Abbreviation ______________________________________ L-aspartyl-L-phenylalanine L-L-APM methyl ester .alpha.-aspartyl-phenylalanine .alpha.-APM methyl ester .alpha.-L-aspartyl-L-phenylalanine .alpha.-L-L-APM methyl ester L-aspartic acid L-Asp L-phenylalanine L-Phe D-phenylalanine D-Phe DL-phenylalanine DL-Phe L-tyrosine L-Tyr L-phenylalanine methyl ester L-PM L-phenylalanine monomethyl L-Phe.CH.sub.3 SO.sub.4 H sulfate ______________________________________
Conventional L-phenylalanine syntheses produce product whose quality is low due to the presence of two impurities. For example, when L-Phe is prepared by a fermentation, it is usually contaminated with L-Tyr, also present in the fermentation broth, and with D-Phe, an optical isomer of L-Phe.
It is known that L-Phe can be selectively crystallized from L-Tyr by derivatizing the carboxy or amino terminus of L-Phe (for example with L-phenylalanine 1 sodium 5 hydrate as disclosed in Japanese Patent Laid Open No. 60-13746 or L-phenylalanine 1/2 sulfate 1/2 hydrate as disclosed in Japanese Patent Laid Open No. 56-79652). However, the solubility of crystals of these L-Phe derivatives increases during crystallization resulting in losses of product into the mother liquor and thus decreased yields. In order to achieve higher yields, crystallization must be performed at high concentrations of L-Phe. As a result, the force necessary to stir the aqueous slurry increases. Additionally, impurities in the mother liquor can adhere to the isolated crystals.
D-Phe is produced as a result of racemization of L-Phe under the high-temperature and alkali conditions used to treat the fermentation broth. D-Phe is also produced as a result of racemization during manufacturing processes which use L-Phe or L-Phe derivatives as starting materials. D-Phe is also produced during attempts to recover L-Phe from these processes (L-Phe is frequently recovered by removing any derivatizing groups by suitable methods such as hydrolysis).
Unfortunately, D-Phe produced during the manufacture of L-Phe is difficult to separate from L-Phe under common crystallization conditions, because D-Phe forms a pair with L-Phe to form a DL-Phe crystal. Since the solubility of DL-Phe is generally far lower than that of the L-Phe, it is difficult to isolate L-Phe.
Various methods have also been studied for recovering L-Phe from manufacturing processes which prepare .alpha.-APM. During the production of .alpha.-APM, L-Phe is methyl-esterified and the resultant L-PM is condensed with L-Asp whose amino group is protected to produce N-protected-.alpha.-L-L-APM. Thereafter, the protecting group is removed yielding .alpha.-L-L-APM.
To obtain L-PM, L-Phe is esterified with methanol in the presence of an acid such as hydrochloric acid or sulfuric acid; the resultant acidic reaction solution is then neutralized with a suitable base in the presence of water; and subsequently L-PM is extracted with a water-immiscible organic solvent such as toluene. In this method, any L-Phe which is not esterified and L-Phe which is produced as a result of the decomposition of L-PM during the neutralization and extraction steps dissolve into the extracted water layer. In addition, any L-PM not extracted into the organic solvent is also dissolved therein.
Since L-Phe is a relatively expensive starting material, it is desirable to recover it from industrial processes and reuse it as a starting material. For example, in the process of preparing .alpha.-APM, L-Phe is recovered from the mother liquor after .alpha.-APM has been crystallized (Japanese Patent Laid Open No. 63-159355; Japanese Patent Laid Open No. 57-130958 etc.). However, no economic or simple method has yet been found to recover the dissolved L-Phe from the above mentioned extracted water layer.
L-Phe is dissolved in low concentration and thus the extracted water layer has to be concentrated before L-Phe can be recovered by crystallization. Since a large amount of energy is required to concentrate the extracted layer, the recovery of L-Phe becomes economically unfeasible.